This invention relates to linear actuators and more particularly, to an improvement to such an actuator for providing noise free operation and for positively stopping a lead screw to which it is attached. While the invention is described with respect to its application in a lift chair, those skilled in the art will recognize the wider applicability of the invention.
Linear actuators are well known in the art and have a variety of uses. Typically, the actuator includes a reversible motor, a threaded, elongate lead screw, and a gear mechanism for rotating the screw in an appropriate direction depending upon motor rotation. In certain applications the screw is connected to a member which, in turn, moves in a desired direction as the lead screw turns in one direction or the other. One such member, for example, is a tube having a nut at one end through which the lead screw is threaded. The linear direction of tube movement is thus relative to the direction of screw rotation. It is possible, particularly in long tubes, for the end of the screw threaded through the nut to contact the inner surface of the tube during actuator operation. While such contact does not render the mechanism inoperative, it does make it noisy. Such noise is a nuisance and undesirable in applicational use.
In addition, linear actuators typically are equipped with limit switches which act to disrupt power to the motor when either extreme of directional movement of the tube along the screw is reached. Occasionally, these switches fail or are mis adjusted. If the motor is rotating the screw in the appropriate direction at the time of such limit malfunction, it is possible for the tube to be completely backed off of the lead screw. When this happens, the mechanism to which the tube is connected fails. If the mechanism is being used, for example, in a lift chair, i.e. a chair that can be raised and lowered to facilitate someone getting into and out of it, the result could be injury to the user.
Heretofore, various approaches have been made to attempt to resolve these problems. For example, in U.S. Pat. Nos. 3,587,796 to Nestvogel, and 3,277,736 and 3,277,737 to Goodman, pins and/or bolts and washers are attached to the end of a lead screw positioned in a tube. While the pin and washer do effect a stop, they have certain drawbacks. If they contact or scrape along the inner wall of the tube, they produce noise. They can be knocked loose and displaced. Also, use of the pins requires a drilling operation as part of the manufacturing process which increases product cost. The bolt and washer require drilling and tapping. All of these operations must be accomplished after assembly of the screw in the tube, making the construction difficult and expensive.
Therefore, it is an object of the present invention to provide an improved but low cost linear actuator mechanism used with a tube guide or the like in which one end of the lead screw is inserted.
It is also an object of the invention to provide a linear actuator wherein contact between the screw and the tube is prevented so to provide for relatively noiseless operation of the mechanism.
It is a further object of the invention to provide a linear actuator which is manufactured without requiring drilling or tapping operations, thus providing a simplified solution to the problem of noise while providing a positive, physical stop for the mechanism.
In addition, it is an object of the invention to provide a safety feature on lift chairs or the like so to insure safe operation of the chair.
In accordance with this invention, generally stated, a linear actuator mechanism is provided having a lead screw connected to a drive tube so one end of the screw is received within the tube. A reversible motor selectively rotates the screw in one direction or the other with the length of the screw within the tube increasing or decreasing depending upon which direction the screw is rotated. A bushing is attached to the end of the screw received in the tube. The bushing serves first to prevent the end of the screw from contacting the inside of the tube to eliminate noise during actuator operation; and second, as a positive stop to prevent the lead screw from being inadvertently run out of the end of the tube. The bushing mounts on the screw and one face of the bushing has a recess formed in it which a washer is sized to fit. The washer supports the bushing, enabling the bushing to provide the positive stop. A retainer ring is also installed on the screw to attach the bushing to the lead screw. The entire lead screw assembly is accomplished externally of the tube.
Other objects and features will be in part apparent and in part pointed out hereinafter.